To reduce the unit cost of semiconductor products and increase production, semiconductor manufacturers have increased semiconductor wafer size. 300 mm wafers are now commonly used, and manufacturers are planning migration to 450 mm systems. This migration introduces new mechanical issues, or exacerbates known issues that were not previously considered significant.
One of these issues is wafer warpage. It is common to observe wafer thickness variation in larger wafers. For example, a spun-on layer may be slightly thicker at the wafer center than at the wafer edge. The absolute thickness variation (in μm) of the wafer tends to be larger if thicker layers are deposited on the wafer. The inventors have observed that in relatively large wafers (e.g., 300 mm and 450 mm), a wafer having a high thickness variation is likely to warp into a concave shape. The warpage may cause greater variation in the height of the wafer top surface than the variation in the thickness of the deposited layer. Wafer warpage can introduce unacceptable results. For example, if the wafer warpage raises the outer edge portion of the wafer, then the exposure light used in a photolithographic process may be out of focus when patterning dies near the outer edge. These focusing problems are particularly disadvantageous for technologies having a critical dimension of 45 nm or smaller. It has been estimated that slight variations in wafer flatness can consume 50% of the critical lithography depth of focus budget at 45 nm. Also, residual stresses in a warped wafer have been observed to result in cracks in the wafer.
Improved methods are desired to avoid undesired effects of wafer warpage.